The present invention is related to a method for producing ophthalmic lenses, especially contact lenses, with an enhanced product quality and yield. In particular, the present invention is related to a method for facilitating mold separation and lens removal from a mold, preferably a reusable mold, in a cast-molding process of ophthalmic lenses (preferably contact lenses), thereby enhancing the quality and yield of produced ophthalmic lenses (contact lenses).
Contact lenses can be manufactured economically in a mass production manner by a conventional cast-molding process involving disposable molds (e.g., PCT published patent application No. WO/87/04390, EP-A 0 367 513, U.S. Pat. No. 5,894,002) or by the so-called Lightstream Technology™ (Alcon) which involving reusable molds and curing a lens-forming composition under a spatial limitation of actinic radiation (U.S. Pat. Nos. 5,508,317, 5,583,163, 5,789,464, 5,849,810, and 8,163,206). The Lightstream Technology™ (Alcon) is an improved cast-molding process for cast molding of hydrogel contact lenses with high precision, fidelity and reproducibility and at low cost. A critical step in the production of lenses using disposable or reusable molds is mold opening and lens releasing from the mold without damaging the lens. Subsequent to the completion of the contact lens molding process, the polymerized lens is in the swollen state in an organic solvent before the organic solvent being exchanged into water and has very low stiffness and toughness. Further, the polymerized lens tends to strongly adhere to the mold. As such, during mold opening and lens demolding (i.e., removing the contact lenses from the mold), cracks, flaws and/or tears may occur in the lenses or in the worst case the contact lenses even break totally. Contact lenses having such defects have to be discarded and lower the overall production yield.
Several methods have been developed or proposed for mold opening and lens demolding. One method for releasing lenses is to hydrate the lens, namely, a lens-in-mold assembly after mold separation is placed in a hydration tank filled with water. Often hydration alone does not release the lenses from the molds. The lenses must then be gently removed from molds by hand. Such hand-assisted lens removal increases the likelihood of lens damage. U.S. Pat. No. 5,264,161 discloses an improved method for releasing a lens from a mold, in which surfactants are added to the hydration bath to facilitate the release of lenses from molds. However, the utilization of surfactants in a hydration bath does not provide a more effortless mold separation. Lens damage incurred during mold separation may not be minimized by hydrating lenses.
Another method of lens release is to incorporate surfactants as internal mold releasing agents into molds themselves as illustrated by U.S. Pat. No. 4,159,292. Incorporation of internal mold releasing agents in molds can decrease adhesion between lenses and molds. However, when a mold is used repeatedly, surfactants as internal mold releasing agent can be exhausted by exudation.
A further method of lens release is to apply external mold releasing agents (e.g., surfactants) in the form of a film or coating onto to the molding surfaces of a mold (e.g., those disclosed in U.S. Pat. Nos. 4,929,707 and 5,542,978 and PCT publication No. WO03095171). When external mold releasing agents are used, a portion of the agents used for treating the molding surfaces of the mold can migrate to the surface and interior of the polymerized lens.
A still further method of lens release is to incorporate internal mold releasing agents into a lens-forming composition for making contact lenses. The internal mold releasing agent can be a surfactant (U.S. Pat. Nos. 4,534,916, 4,929,707, 4,946,923, 5,013,496, 5,021,503, 5,126,388, 5,594,088, 5,753,730), a non-polymerizable polymer (U.S. Pat. No. 6,849,210), or a phospholipid (U.S. Pat. No. 8,440,735). By incorporation of an internal mold releasing agent in a lens-forming composition (or lens formulation), the adhesion between molds and lenses may be reduced, a relatively smaller force may be required to separate mold, and lenses may be removed from molds with less effort. A portion of the internal mold releasing agent need migrate to the interface between the lens-forming composition and the mold in order to be effective to reduce the adhesion between molds and lenses. However, the migration speed of the internal mold releasing agent from the lens-forming composition to the interface between the lens-forming composition and the mold may not be fast enough to effectively reduce the adhesion between molds and lenses.
Therefore, there is a need for an improved method for making contact lenses, in which mold separation force and lens-mold adhesion can be substantially reduced so as to achive higher lens quality and higher production yield.